The invention relates generally to the fields of ion exchange, ultrafiltration and electrodialysis and, more specifically, to the methods for utilizing ion exchange, ultrafiltration and electrodialysis to produce high purity colloidal silica and/or potassium hydroxide.
Colloidal silica is a suspension of very small, spherical particles of amorphous (not crystalline) silica suspended in water. The material is colloidal in that the silica particles do not settle out of the solution. The silica particles generally range in size from about 8 nanometers to a (maximum size of about 300 nanometers, or, more, beyond which the silica ceases to be collidal and begins to settle out of solution. Colloidal silica has been used for a variety of purposes, including precision casting, as a lining for molds, as a frictionizing (non-skid) agent, and in a variety of medical and pharmaceutical uses such as toothpaste, dental castings and drug delivery systems.
One relatively recent use which has arisen for colloidal silica is in the polishing of semiconductor materials, such as the silicon wafers used in computer chips, by a process known as Chemical Mechanical Planarization (CMP). CMP involves the polishing of semiconductors and chips using very small abrasive particles of silica alumina, ceria or other materials in a slurry or suspension with a chemically active carrier solution. Colloidal silica has also recently come into use in polishing other items such as hard disk drives, electronic memory devices and raw silicon oxide wafers to accomplish the rough polishing of chips prior to CMP.
The electrical performance of finished semiconductor chips can easily be affected by contaminants to which the semiconductor wafers are exposed during processing. Such contamination can be in the form of discrete particles and water soluble or dispersed organic and inorganic impurities. In particular, the use of silica sols that are contaminated with trace transition metals, alkali and alkaline earth metals, aluminum, and other metals causes difficulties when used in wafer polishing. Sodium, potassium, alkali and alkaline earth metals such as calcium, magnesium, and transition metals such as iron, copper, manganese, nickel, and zinc are particularly troublesome. In general, any transition metal from groups IB, IIB, IIIB, IVB, VB, VIB, and group VIII of the Periodic Table of Elements, if present in high enough concentrations, can cause difficulties in the final products manufactured with silica sols containing these contaminants.
An additional problem of metal contaminants is that many of these substances have much higher diffusivities in both silicon and silicon dioxide than do the more conventional dopants, such as phosphorus and boron. As a result unpredictable electrical properties can result when silicon wafers are contaminated with these metals. For example, alkali metals such as lithium, sodium, and potassium cause shifts in electrical properties (threshold and flat-band voltages) when incorporated into semiconductor devices.
Currently used colloidal silica CMP slurries generally include a relatively high amount of contaminating sodium, which is very difficult to remove. The contaminating sodium causes defects in the chips in the later manufacturing processes. Accordingly, there is a need for an improved method for producing very low sodium content colloidal silica for use in these applications. There is also a need for an improved method of producing high purity potassium hydroxide which is also an additive used in some CMP slurries.
One method which has been developed in an attempt to produce a low sodium potassium stabilized silica sol is disclosed in U.S. Pat. No. 4,915,870 to Jones. The method disclosed by Jones uses an acid sol process to produce a silica sol having a sodium concentration of less than 150 ppm using commercially available KOH to stabilize the pH of the sol. However, while the method disclosed by Jones is suitable for producing silica sols having a sodium concentration of less than 150 ppm, it would be preferable to produce silica sols having much lower sodium concentrations for use in electronics applications.
One method of producing a colloidal silica sol having a substantially uniform particle size is disclosed in U.S. Pat. No. 3,789,009 to Irani. The sols are produced by adding simultaneously an alkali metal silicate and a cation exchange resin to a heel of water containing preformed colloidal silica particles at a pH within the range of 8-11, at a temperature of 60-150 degrees Celsius, and at a rate below that at which nucleation occurs. The cation exchange resin is added to the heel to remove the alkali metal cations from the alkali metal silicate causing the silicic acid to polymerize onto the heel nuclei to form large silica sols. Since the polymerization rate is similar for all of the uniform seed particles, the resulting larger sols also have a relatively uniform size distribution. By controlling the amount of alkali metal silicate and resin added it is possible to produce colloidal silica particles having a uniform size of between 8 nm and 200 nm. In this process, preferably the silicic acid concentration is maintained below the nucleation point to avoid the formation silica particles not sharing in the uniform size. Other processes to produce such sols are well known in the art.
Therefore, it is an object of the present invention to provide a system and method for producing very low sodium colloidal silica suitable for use in electronic applications.
It is another object of the present invention to provide a process for producing colloidal silica particles having a uniform shape and size.
It is yet another object of the present invention to provide a method for producing very high purity potassium hydroxide for use in the CMP slurry, and other uses.
The above objectives are accomplished according to the present invention by providing a system and method for producing high purity colloidal silica which includes the steps of providing a quantity of potassium silicate; subjecting the quantity of potassium silicate to an ion exchange process to remove a first portion of potassium therefrom to produce a quantity of colloidal silica; and subjecting the quantity of colloidal silica to ultrafiltration to remove a portion of sodium therefrom, producing a quantity of high purity colloidal silica.
In a further preferred embodiment of the present invention, the ion exchange resin which has previously been utilized to remove potassium from the potassium silicate may be utilized in the production of high purity potassium salts. To produce a very low sodium, high purity potassium salt, the ion exchange resin may be regenerated by contacting it with an acid to produce a quantity of potassium salt, which may be further purified by evaporation, crystallization and/or ultrafiltration. The low-sodium potassium salt produced is further processed by electrodialysis and/or electrolysis in order to produce a low-sodium potassium hydroxide, suitable for re-addition to the low-sodium colloidal silica to produce a very low sodium potassium hydroxide stabilized CMP silica slurry.
Other objects, aspects, and advantages of the present invention will be apparent to those skilled in the art from a reading of the following detailed disclosure of the invention.